This invention relates to an improvement in a method and apparatus for accomplishing lead bonding to electrical circuits.
In the manufacture of semiconductor devices, a large number of circuits are formed on a single semiconductor slice and certain points in the various circuits are connected together by an electrical conductor having a very small cross-sectional area. Such a wire may be as small as one circular mil. There are various ways in which the ends of the wires may be attached to the circuit point, such as by thermocompression, eutectic bonding, or ultrasonic bonding. The wire in such methods is typically extended through a hollow, capillary type tool which is lowered to the circuit point to which the wire is to be attached. One technique which is sometimes used in these methods, and particularly in the thermocompression type bonding, is to form the extended end of the wire into a ball having a diameter greater than the diameter of the wire to which it is attached. The capillary tool is then used to press this balled end of the wire against the circuit point to which it is to be attached and the capillary tool and circuit point and wire are heated to a predetermined temperature to effect the bond. The capillary tool is then raised, while simultaneously extending additional wire out of the tool, and is moved to a second circuit point. The capillary tool is pressed downwardly to mechanically bond a second portion of the wire to a second circuit point and is then raised again. The portion of the wire extending between the capillary tool and the second circuit point is then severed, either by simply putting the wire in tension until it breaks, or by flame heating it until it is severed. One disadvantage of the first technique is that the length of wire left extending from the capillary tool must then be heated to form another ball. This requires an additional step in the operation and since it is desirable to minimize the number of steps in such operations, this is a disadvantage. Also the capillary tool must move in only one predetermined direction while breaking the wire. This limits its mobility and increases the time required to perform a large number of lead bonding operations. A disadvantage in the flame severing type of operation is that a pigtail is left on the circuit board. The pigtail is a small portion of the wire which extends upwardly from the second circuit point to the end of the wire where the flame severed it. This portion must be removed by a time consuming manual operation to prevent it from contacting any other part of the circuit.